Boron nitride (BN) is a most interesting III-IV compound from both the practical and scientific viewpoints. Boron nitride is characterized by three different crystal structures: hexagonal (hBN), wurtzite (wBN) and cubic zincblende (zBN). It is interesting that the physical properties of the boron nitride phase characterized by the cubic zincblende crystal structure are comparable to diamond in that the cubic zincblende boron nitride and diamond have low densities, extremely high thermal conductivities, and large resistivities. In addition, the cubic zincblende boron nitride and diamond have similar tribological properties and are relatively inert chemically.
There has been much research directed toward the growth of diamond thin films for various purposes. Therefore significant effort has also been directed to growing these cubic zincblende boron nitride films for the same purposes. This is not only because of the similarities between the two materials in their electrical, thermal and tribological properties, but because the cubic zincblende boron nitride may also prove to be an attractive substrate for subsequent diamond growth due to the small mismatch in crystal lattice constants. As with the diamond films, previous attempts at depositing cubic boron nitride films have failed to produce the desired homogeneous, single-crystal, and epitaxially oriented films.
Pulsed laser deposition of thin films has recently been demonstrated to be a useful technique for preparation of thin films of a wide variety of materials including polymers, semiconductors, superconductors, and nonlinear dielectrics. Typically, when utilizing laser deposition techniques, a substrate of appropriate material is maintained at an elevated temperature opposite to a target having a composition the same or similar to the desired thin film. A focused pulsed laser beam, usually from an excimer laser source, is incident on the target at an angle of approximately 45.degree.. The deposition is generally performed in a vacuum or other appropriate atmosphere such as flowing oxygen in the case of the copper oxide superconductors.
Advantages of the laser deposition method over other deposition techniques, such as evaporation, include a faster deposition rate, the requirement for only a single target, and the ability to deposit materials possessing high boiling point temperatures, such as refractory materials. Advantages over sputtering deposition methods also include the requirement of only a single target, as well as the preservation of material composition from the target to the film. It is therefore advantageous to use laser deposition techniques for the formation of thin films of materials. In particular it would appear to be advantageous to use these laser deposition methods for the formation of thin films of materials such as boron nitride.
In summary, it is desirable to provide cubic boron nitride thin films, and particularly, to provide a method for forming these thin films, wherein the resulting thin films of cubic boron nitride are essentially single-crystal, homogeneous and epitaxially oriented with the underlying substrate.